1. Field of the Invention
This invention relates to foamable thermoplastic resin compositions, coextruded rigid foam articles, and processes for making coextruded rigid foam articles, and more particularly relates to foamable thermoplastic resin compositions having a vinyl chloride polymer, a graft copolymer, a rubber-free copolymer, and a chemical blowing agent, coextruded rigid foam articles made therefrom, and processes for making such coextruded rigid foam articles.
2. Description of the Related Art
Polyvinyl chloride compositions comprising graft copolymers having a superstrate of alphamethyl styrene-unsaturated nitrile-unsubstituted styrene and a substrate of butadiene rubber, are known for making non-foam articles. Such compositions have exhibited increased heat distortion temperatures due to the presence of the graft copolymer, but such compositions have typically lacked the melt strength required for making foamed articles, and thus have not contained blowing agents therein or been made into rigid foam articles. Additionally, the addition of the heat distortion temperature increasing graft copolymer into foamable thermoplastic resin compositions would not be obvious to one skilled in the art due in part to the lack of a need for increased heat distortion temperature in typical uses for rigid foam vinyl chloride articles, for example, foam drain pipe, waste pipe, vent pipe and foam sheet for construction.
Foamable thermoplastic resin compositions comprising vinyl chloride polymer, methyl methacrylate-styrene-acrylonitrile terpolymer and a blowing agent are known, see French published application serial no. 89/05,537, filed Apr. 26, 1989, by inventors Jacky Patric Dominique Des Champs and Marc Loibl, but such compositions have typically exhibited higher levels of viscosity than are desired for coextrusion processes of non-foamed resin skin and the foamable compositions which due to pressure build ups and excessive shear in such processes can lead to heat build up of the compositions due to the working torque resulting from the shear, and leading to premature foaming of the materials and possible degradation of the vinyl chloride polymer resulting from the temperature increase.
Foamable thermoplastic compositions comprising vinyl chloride polymer, butylacrylate rubber-based graft copolymers and polymethyl methacrylate rigid polymer have been used in coextrusion processes for making rigid foam structures, but such materials typically exhibit undesirably low impact strengths, and elongated cell structures in the foam due to poor melt strength rather than circular/spherical cell structures.
Accordingly, there is a need to provide foamable thermoplastic resin compositions, which exhibit enhanced melt strength, reduced viscosity, reduced processing sensitivity and desired formation of spherical cell structures, impact strength and rigidity in coextruded rigid foam structures formed therefrom by coextrusion processes.